It is well known that the speed and torque of a polyphase induction motor is a function of the frequency of the alternating power that is supplied to the motor and the voltage that is applied to the windings.
It is also well known that the motor can operate at either a synchronous speed, the same frequency as the power to it, and an asynchronous speed, higher or lower than that frequency. It operates at the synchronous speed when the motor is under no load and at the asynchronous speed when the motor is under load or being driven. The speed difference is known as slip, the magnitude of which dramatically impacts the motor's efficiency and performance.
Therefore, the typical polyphase motor is given requirements for maximum slip by the manufacturer, typically about .+-. Hz., depending on the motor design and whether it is motoring or regenerating, or braking. To develop maximum torque and maximize motor efficiency, the slip under load conditions should be held at that figure. For instance, if a two-pole motor is powered from a 60 Hz. source during motoring, the speed of rated torque would be about 3540 r.p.m., that being a positive slip of +1 Hz.
Following a converse concept, but using the same criteria, if the motor is moving at a speed which is greater than the frequency, power can be supplied from it or regenerated back to the source. The slip also should be maintained within those limits in this case. But, for regeneration or braking, the speed of the motor may be, for instance, 3660 at rated torque, in which the slip is -1 Hz., this being negative slip.
So, it is not surprising that many techniques have been tried to control slip precisely; however, most have met with less than desirable results because they are too costly, too complicated or do not provide good performance.
In an elevator slip control it is particularly important and demanding because motor performance there must be superior to that in most other applications. For instance, for a comfortable ride the motor must be accelerated and decelerated smoothly, without vibration and noise; yet, for system speed it must be fast. It should also be efficient, which means it should regenerate power, and, naturally, it must be operated in such a way as to provide precise car positioning at the floors. Most important, the motor must often be operated at near zero speed, at which precise frequency control is critically important for smooth performance.